v0.5: performance & git worktrees

Not many changes here, but a nasty bug was fixed, worktrees are handled well now and performance of adding a build went up by an order of magnitude or so (depends on GCC version in use, project size and how coverage is collected).

General

• Create index on files table of the database. This bumps database version even though its schema doesn't actually change. The change grows database size a bit, but makes addings new reports significantly faster.

Core and CLI

• Handle git worktrees (config and database are looked up first in parent's .git now).
• Use "current_working_directory" key of gcov's JSON.
• Take advantage of --stdout option of gcov if present. new-gcovi works significantly faster thanks to it and with less I/O.
• Fixed one use-after-free.

Vim-plugin

• Update Vim plugin for newer fugitive API.

Building

• Remove -Werror from Makefile.
• Make it work better with clang.